Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/classes/kt2391.kt

@@ -0,0 +1,19 @@
+public trait LoggerAware {
+    public val logger: StringBuilder
+}
+
+public abstract class HttpServer(): LoggerAware {
+    public fun start() {
+        logger.append("OK")
+    }
+}
+
+public class MyHttpServer(): HttpServer() {
+    public override val logger = StringBuilder()
+}
+
+fun box(): String {
+    val server = MyHttpServer()
+    server.start()
+    return server.logger.toString()!!
+}